In the field of driver circuitry used for driving the coils of motors of the DC brushless type, it is well known to control the drive current applied to the driven coil using feedback from the load current that is supplied to the driven motor coils. Such feedback control allows precise control of motor torque and motor position, which is especially useful in certain motor applications, such as disk drive motors in modern computers.
A typical conventional technique for sensing the load current in a motor control system is the use of a small series sensing resistor, connected in series with the motor coils being driven. Measurement of the voltage drop across the resistor will, of course, indicate the magnitude of the current through the driven coil at each particular instant.
An example of the use of a sensing resistor in the field of motor control, is shown in FIG. 1 hereof. In this example, which is described in detail in U.S. Pat. No. 5,306,988, issued Apr. 26, 1994, assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by this reference, a brushless DC motor includes stator (or, alternatively, rotor) coils 2, 3, 4, connected to a center tap 6 in the conventional "Y" arrangement. Driver circuit 5 includes, for each of motor coils 2, 3, 4, a pull-up transistor 14 and a pull-down transistor 15. The gates of pull-up transistors 14, 14', 14" (for motor coils 2, 3, 4, respectively) are driven by lines from a commutation sequencer. The gates of pull down transistors 15, 15', 15" (for motor coils 2, 3, 4, respectively), are driven by a respective power switch 7, 7', 7", under control of the commutation sequencer. Sensing resistor 9 is connected between the sources of transistors 15, 15', 15" and ground; the voltage across resistor 9 is amplified by constant gain circuit 8, and applied to error amplifier 10 along with a reference voltage V.sub.IN. The output of error amplifier 10, which is a voltage proportional to the differential voltage at its inputs, is provided to each of power switches 7, 7', 7". During a commutation sequence, the one of pull-down transistors 15 that is to be turned on will have the output voltage of error amplifier 10 applied to its gate. As such, the current sensed by resistor 9 determines the amount of low side drive applied by driver circuit 5.
Another implementation of current sensing by way of a sensing resistor is described in U.S. Pat. No. 5,204,594, issued Apr. 20, 1993, assigned to SGS-Thomson Microelectronics, Inc., and incorporated herein by this reference.
However, it has been observed that the use of a sensing resistor is disadvantageous due to its necessary power dissipation (i.e., the square of the load current times the resistance value), and also because the available voltage drop across the driven coils is reduced from that which is otherwise available, thus reducing the maximum available drive to the coils.
By way of further background, copending application Ser. No. 07/890,945, filed May 29, 1992, entitled "Circuit for Providing Drive Current to a Motor Using a SenseFET Current Sensing Device and a Fast Amplifier", assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by this reference, describes another approach for sensing the drive current applied to motor coils. According to this technique, a so-called "sensefet" is the high side driver which drives the coil from a source output and which also provides a current sensing output; the sensing output and the source output of the sensefet are applied to differential inputs of an amplifier to provide a feedback signal to the motor control system.
Another conventional technique utilizes current mirror sensing to measure the load current. In this approach, mirror transistors are connected substantially in parallel with drive transistors in the motor control circuit, so that the current through the drive transistor is mirrored in the mirror transistor. Usually, the mirror transistor is significantly smaller (in width/length ratio) than its associated drive transistor, so that the current therethrough is a fraction of that through the drive transistor.
According to one implementation of the current mirror sensing scheme, a resistor is in series with the source/drain path of the mirror transistor, so that the voltage across the resistor is proportional to the current through the mirror transistor, and thus proportional to the current through the corresponding drive transistor. According to another technique, described in U.S. Pat. No. 4,827,207, issued May 2, 1989, and incorporated herein by this reference, a voltage regulator circuit is used to equalize the drain voltage of the primary and mirror transistors to one another; in this approach, the sensing resistor is included within the feedback loop of the voltage regulator.
When applied to a polyphase brushless DC motor, the conventional current mirror sensing approach individually senses current driven through each of the push-pull legs of the driver circuit. FIG. 2 illustrates such an arrangement, where rotor coils 2, 3, 4 are driven by power transistor pairs 14/15, 14'/15', and 14"/15" as before. In each driver leg, a mirror transistor 16 is provided that has its gate and source biased in common with the gate and source of an associated low side drive transistor 15. Resistive load 17 is provided in series with the source-drain path of mirror transistor 16, across which a voltage reading may be made. As noted above, it is conventional to have the width/length ratio of mirror transistor 16 be much smaller than that of its associated low side driver transistors 15, so that the current conducted through mirror transistor 16 is proportional but much smaller than that conducted by its low side driver transistor 15, to minimize power dissipation in the sensing leg. As is evident in FIG. 2, each leg of the driver circuit includes such a current mirror sensing arrangement.
In an arrangement such as FIG. 2, it is important to obtain information regarding the load current (as sensed by mirror transistor 16 and sensing load 17) at all times during the commutation cycle, including during commutation switching. As such, the mirror leg must accurately measure the varying current levels that are produced during the various phases and times within a phase. It has been observed, however, that measurement inaccuracies result from mirror sensing such as that illustrated in FIG. 2, considering that the transients introduced by commutation between phases upset the accuracy of the mirror leg.
By way of further background, it is well known in the art that it may be useful to operate a brushless DC motor in bipolar mode (i.e., with two coils driven in each phase) during startup of a motor, and then to switch to unipolar mode (i.e., only one coil driven in each phase) once a desired operating speed is reached. This operation is beneficial since the unipolar operation reduces the effects of back emf induction in the driven coils at higher rotational speeds. As is known in the art, back emf in a driven coil reduces the available driving voltage that can be applied to the coil, which reduces the drive current to the coil and thus limits the torque of the motor.
It is therefore an object of the present invention to provide an improved circuit and method for sensing load current in the driving of a brushless DC motor.
It is a further object of the invention to provide such a circuit and method that is less susceptible to commutation transients.
It is a further object of the invention to provide such a circuit and method that is may be applied to a motor driver that utilizes both bipolar and unipolar commutation modes.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.